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Masood Ahmad Shah

This blog contains a summary of my research readings and thoughts in system and network engineering. View Masood Shah's profile on LinkedIn

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IGP competition ISIS or OSPF

I understand that this question is a lot more complex than a simple yes or no since factors like design and routing policy will certainly affect the protocols behavior. It's really difficult to decide and get information on what the top network service providers are using for their IGP? I'm trying to build a case for switching from OSPF to IS-IS and here are my findings:

• In OSPF all areas must be directly linked to area 0, and the backbone area must also not be segmented.
• With IS-IS, the backbone area can be more easily extended since all L2 routers need not be linked directly together.
• With regard to CPU use and the processing of routing updates, IS-IS is more efficient than OSPF.
• In IS-IS, one LSP is sent per IS-IS router in each area (including redistributed prefixes [routes]), compared to the many OSPF LSAs that would be sent.
• Not only are there fewer LSPs to process, but the mechanism by which IS-IS installs and withdraws prefixes is less processor intensive.
• In IS-IS, the entire SPF table is not refreshed periodically like OSPF, which does so every 30 minutes by default.
• L2 routers are similar to OSPF backbone routers, and the set of L2 routers (including L1/L2 routers) and their interconnecting links make up the IS-IS backbone, similar to area 0 in an OSPF network.

• Some routers, called L1/L2 routers, belong to both area types.
• Unlike OSPF, IS-IS routers are not required to be connected to a contiguous backbone area. In fact, the backbone area can also be segmented in IS-IS.
• IS-IS uses the concepts of router levels, which is similar to OSPF areas. L2 routers are similar to OSPF backbone routers, and L1/L2 routers are analogous to OSPF ABRs.
• With IS-IS, there is no restriction that all backbone routers (level 2 routers) be contiguous such as the backbone area of OSPF.

Reference: Jeff Doyle, “Routing TCP IP”

Published Thursday, January 22, 2009 5:28 PM by jahil

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